1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor apparatus and a method of forming a viscous liquid layer that include a process of disposing viscous liquid on a substrate.
2. Description of Related Art
Semiconductor devices used for basic communication equipment, high-end computers and so on face increasing demands for higher speed and higher performance. To meet these demands, it is necessary for the semiconductor devices to achieve higher pin counts and better electrical property. A solution to achieve these objectives is Ball Grid Array (BGA) semiconductor apparatus.
Wire bonding technology that places a chip with its circuit surface facing up and connects it to a terminal with a thin gold wire has been used as a method to electrically connect a semiconductor chip and a packaging substrate. The BGA semiconductor apparatus, on the other hand, uses flip-chip technology that places a semiconductor chip with its circuit surface facing down and connects it to a packaging substrate with a solder or gold terminal (bump) to make electrical connection.
This technology is superior to the wire bonding technology in having better electrical property due to shorter wire length and thus allowing application to higher-speed and higher-density devices. Further, since this technology enables to place pins two-dimensionally under the semiconductor chip, it can achieve higher pin counts of as many as several thousand easily.
In the flip-chip technology, a technique for applying an adequate amount of flux onto a principal surface of a semiconductor chip is extremely important so as to accurately place a bump such as a solder at a predetermined position of a semiconductor chip. Various flux application methods are therefore proposed as described in Japanese Unexamined Patent Publication No. 06-124954 and 11-186329, for example.
FIGS. 4A to 4F are schematic flow charts to describe a method of applying flux onto a semiconductor chip and a method of placing a bump according to a conventional technique.
Flux is viscous liquid that serves to fix solder balls or the like, which are described later, so as to form bumps at accurate positions. As shown in FIG. 4A, a mask 101 that is used when applying flux has a meshed opening 101a that allows flux 103 to pass through the mask 101 perpendicularly to its principal surface. On the principal surface of a semiconductor chip 102 to be coated with the flux 103, given number of electrodes 102a with a diameter of about 100 μm are formed.
Referring first to FIG. 4A, the mask 101 for screen printing is placed on the principal surface of the semiconductor chip 102 having the electrodes.
Referring next to FIG. 4B, the flux 103 is applied onto the surface of the semiconductor chip 102 where the electrodes 102a are exposed by using the mask 101 for screen printing. Specifically, this process places the flux 103 at one end of the mask 101 which is not on the meshed opening 101a and then brings it over the mask 101 by using a squeegee 104. It moves the squeegee 104 to the other end of the mask 101 horizontally at a predetermined speed with the edge of the squeegee 104 in contact with the mask at predetermined angle and pressure. The flux 103 is thereby applied onto the semiconductor chip 102 through the meshed opening 101a. 
Referring then to FIG. 4C, the mask 101 is removed. The semiconductor chip 102 is thereby coated with the flux having a thickness of about several tens of μm.
Referring then to FIG. 4D, a solder ball 106 is tentatively fixed on the flux 103 by using a ball mounter 107. The diameter of the solder ball 106 is substantially the same as the diameter of the electrode 102a. 
Referring finally to FIG. 4E, the solder ball 106 is melted by heat treatment to form a solder bump 108. The heat treatment is performed at a higher temperature than a solder melting temperature. The heat treatment diffuses the component of the flux 103 into the air like water vapor. After the heat treatment, the remaining flux 103 is removed by washing or the like. A series of processes are thereby completed.
Japanese Unexamined Patent Publication No. 06-124954 proposes a technique that applies flux partially on a substrate surface with exposed electrodes by using screen printing. Japanese Unexamined Patent Publication No. 11-186329 proposes a technique that applies an adequate amount of flux by using a roller and a rubber plate.
An adequate amount of flux application thickness varies by the diameter or pitch of solder balls.
If the flux application thickness is larger than an adequate thickness, connection between the solder ball 106 and the electrode 102a can fail due to the thickness of the flux as shown in FIG. 5A. In such a case, the solder ball 106 can be displaced during reflow to make contact with the adjacent solder ball (see FIG. 5A), which causes a bump bridge (see the solder bump 108a in FIG. 5B) and results in short-circuit. Further, if the flux is too much, it can be left in the gap between electrodes or the like after reflow. This raises the need for a longer washing time. If the flux is still left after washing, it hinders resin sealing.
On the other hand, if the flux application thickness is smaller than an adequate thickness, adhesive force is too weak to hold the solder ball 106 at the position of the electrode 102a as shown in FIG. 5C, which causes displacement to occur (see the solder ball 106a in FIG. 5C). If the adjacent solder balls 106 are in contact with each other, it causes a bump bridge and results in short-circuit.
When applying highly viscous flux onto a substrate, however, the primary problem is technical difficulty in applying it thinly rather than controlling the application thickness.
The technique according to Japanese Unexamined Patent Publication No. 06-124954 is expected to reduce the amount of flux to be used since it applies flux only partially. However, the present invention has recognized that this technique requires a mask for screen printing to apply flux partially for each diameter or pitch of solder balls or the like, which increases costs. Further, this technique requires accurate positioning of a mask, thus not suitable for reduction in the diameter or pitch of solder balls. Furthermore, this technique forms a mask to surround flux after applying the flux, and since the mask has low heat resistance, a problem can occur in a process of melting solder bumps. It is thereby difficult to apply this technique in practice.
The present invention has also recognized that the technique according to Japanese Unexamined Patent Publication No. 11-186329 has a problem that the application thickness of flux changes with time due to abrasion of a roller or rubber plate over time. Further, this technique requires large scale equipment.
Though the problems in the flux that is used in the manufacturing process of BGA semiconductor apparatus are described above, similar problems can occur in general viscous liquid to be applied onto a substrate.